1. Field
The present disclosure relates generally to electric motor systems and, in particular, to controlling electric motor systems. Still more particularly, the present disclosure relates to a method and apparatus for controlling the operation of an electric motor in an electric motor system.
2. Background
An electric motor is a device that converts electrical power into mechanical power. Electric motors are used for various applications. These applications include fans, pumps, tools, disk drives, drills, and other types of devices that may be found in these and other types of platforms.
An electric brushless motor is a commonly used type of electric motor. With electric brushless motors, a controller is configured to change the current in windings in the electric motor. In particular, the current is switched at a frequency that changes the amplitude of the current applied to the windings in phases in a manner that causes the motor to turn. The switching of the current is performed using switches in the form of transistors.
Parameters such as torque and speed may be controlled in an electric motor. These parameters may be controlled by altering the duty cycle. The duty cycle is the percent of time that an object spends in an active state as a fraction of the total time that is considered. With respect to electric motors, the duty cycle may be defined as the ratio between the pulse and the period of a rectangular wave form.
For example, a duty cycle of 0 percent may be no torque. The duty cycle of 100 percent may be a maximum torque that can be generated by the electric motor. The efficiency of the motor is highest when the duty cycle is 100 percent.
However, when the duty cycle is 100 percent, the controllability of the electric motor is reduced. In particular, the control of maintaining the speed of the motor a particular number of revolutions per minute (RPM) may be limited. For example, if the desired operation of the electric motor is a speed of 6000 RPM, a duty cycle of 100 percent may be used to operate the electric motor at the speed of 6000 RPMs in this example.
However, increasing the speed beyond 6000 RPMs is not possible when the duty cycle is already 100 percent. Further, if the load applied to the motor reduces the speed below 6000 RPMs, the speed cannot be returned to 6000 RPMs by increasing the duty cycle since the duty cycle is already at 100 percent. As a result, the controllability of the electric motor is not as great as desired when the duty cycle is 100 percent.
Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues.